Iron containing substrates such as strip steel may be electroplated with tin in large continuous plating machines at steel mills throughout the world. In such machines a large coil of steel sheet unwinds at one end of the machine and proceeds through cleaning and acid pickling stations followed by multiple tin electroplating stations to produce a tin deposit over the steel surface. The tin coating, as plated, exhibits a characteristic smooth matte surface.
The next section of the line is variously known as the “flow-melting”, “flow-brightening”, or “reflow” section. The reflow operation is used to transform the matte deposit to the bright reflective finish typical of tinplate and to produce a thin iron-tin compound layer at the interface between the tin coating and the steel base, thereby improving corrosion resistance. The operation includes the steps of raising the temperature of the tin coating to above the melting point of tin, followed by immediate quenching to impart and achieve the desired properties of the deposit.
In the reflow operation, after the matte tinplate is rinsed, the steel sheet proceeds through a fluxing station. The term “flux” in this context refers to a substance that aids, induces, or actively participates in fusing or flowing. The application of flux is followed by drying and the reflow station itself raises the temperature of the steel to above the melting point of tin. The steel is then quickly quenched in water resulting in a tin surface that has a bright finish. After reflow the steel proceeds through other stations for treatments such as passivation, oiling and rewinding or cutting into sections at the exit-end of the machine.
A uniform, bright finish is achieved without blemishes or discontinuities if all of the above steps are optimally executed otherwise tin oxides or hydroxides as well as “woodgrain” form on the tinplate surface. Woodgrain is a common visual defect that can occur in conduction reflowed tinplate when light tin coating weights are produced in which the surface takes on a non-uniform appearance analogous to that of polished wood. The formation of tin oxides and hydroxides may cause defects in the tin finish during reflow. This defect is observable on the surface of the tin as a white haze. Another common defect is a blue haze caused by acid etching of the tinplate due to over application of a fluxing compound or excess residual plating acid. Many desirable tin electrolytes include acids such as phenolsulfonic acid, sulfuric acid, fluoborate and alkyl sulfonic acids. When an excess of plating acid or fluxing compound is left on the strip due to insufficient rinsing before the flux, it causes a blue haze or over-etching effect. Tin plating lines whose electrolytes are based on phenol sulfonic acid (PSA) do not have a separate flux tank, but instead rely on a low post-plating rinse efficiency to leave an amount of residual PSA on the tinplate surface sufficient to operate as a flux.
However, due to a general migration from PSA to methane sulfonic acid (MSA), a more environmentally sustainable tin plating electrolyte, ex-PSA tin plating lines are typically found to be one rinse-tank short by having to lose one of their rinse-tanks by converting it to a flux tank, required for MSA operation. Insufficient rinsing after plating results in an excessive contamination of the flux solution and a higher propensity for reflow-melting defects, such as haze, over-etching and woodgrain. Accordingly, there exists a need for an improved flux method which inhibits tin oxide and hydroxide formation, acid etching and the formation of woodgrain.